1. Field of the Invention
The present invention relates to an electrodynamic electroacoustic transducer, a diaphragm thereof, and a method of manufacturing the same. More particularly, the present invention relates to an electrodynamic electroacoustic transducer applied to a dynamic headphone or a dynamic microphone, a diaphragm thereof, and a method of manufacturing the same.
2. Description of the Related Art
A dynamic headphone or a dynamic microphone is an electrodynamic electroacoustic transducer that converts an electric signal into a sound wave or converts the sound wave into the electric signal, using the law of electromagnetic induction. FIG. 3 illustrates a cross-sectional view of a dynamic microphone unit. As illustrated in FIG. 3, a microphone unit 50 includes a magnetic circuit in which a disk-shaped pole piece 51 and a magnet 52 to be overlapped are surrounded by a yoke 53. In addition, a gap is formed between the pole piece 51 and the yoke 53. A cylindrical voice coil 55 attached to a diaphragm 54 is arranged in the gap in a vertically movable state.
The yoke 53 is held on an upper portion of a cylindrical unit housing 57. A unit frame 58 having an air chamber provided inside is held on a lower portion of the unit housing 57.
In addition, the diaphragm 54 is covered by a unit cap (resonator) 59 having a plurality of front portion acoustic terminals 59a. 
In the microphone unit 50 configured as described above, if the diaphragm 54 is vibrated by the sound wave, the voice coil 55 vibrates vertically at the same time as the vibration of the diaphragm. As a result, electromotive force is generated in the coil and the microphone unit 50 outputs an audio signal based on the sound wave. Such a configuration of the dynamic microphone is disclosed in JP 2013-141189 A, for example.
FIG. 4 is a cross-sectional view of a state before the diaphragm 54, the voice coil 55, and the resonator 59 are attached to the unit housing 57.
Generally, the diaphragm 54 is manufactured by molding a plastic film under heating and pressure. In addition, the voice coil 55 is attached to the diaphragm 54 (referred to as a diaphragm assembly 56) and the diaphragm assembly 56 is attached to the unit housing 57.
Meanwhile, when the diaphragm assembly 56 is attached to the unit housing 57, an outer diameter of the diaphragm 54 needs to be smaller than an inner diameter of the unit housing 57. For this reason, a predetermined gap (about 0.1 to 0.4 mm) is provided in an outer diameter portion of the diaphragm 54 and an inner diameter portion of the unit housing 57.
In addition, in attachment work, an edge portion of the diaphragm is adhered to a peripheral portion of the unit housing 57 using an adhesive material. For this reason, it is necessary to press the edge portion of the diaphragm from the upper side using a weight until the adhesive material is solidified.
However, if the diaphragm is pressed by the weight, distortion occurs in the diaphragm 54. The distortion causes eccentricity at the center of the diaphragm 54 and the center of the unit housing 57. For this reason, the voice coil 55 contacts the magnetic circuit side and accurate mounting of the diaphragm assembly 56 is obstructed.
In the diaphragm pressed by the weight, the weight does not contact the peripheral portion of the diaphragm uniformly. For this reason, the diaphragm 54 and the unit housing 57 are fixed in a state in which stress is applied to the diaphragm 54. In the microphone, the stress lowers frequency response of a low frequency. In the case of the headphone, the stress generates chattering noise.